The invention relates to a method and apparatus for sealing boreholes in the earth. A major problem facing the nuclear industry which has received considerable attention is how to dispose of radioactive wastes so they will never contaminate the biosphere with radioactivity. It is believed that the disposal of radioactive wastes in geologic formations has the potential of isolating these wastes from man's environment for adequate time periods. Geologic environments exist which have been physically and chemically stable for millions of years, are isolatable from man's environment and have the potential to provide effective barriers between the waste and man's environment for the time periods required. Placement of solidified radioactive waste containing canisters in geologic formations, whether placed individually into boreholes or placed as a group in a rock cavity, will result in a penetration of the geologic environment with a potential for contact with man's environment. In order to assume the containment of nuclear waste by the geologic environment, these penetrations must be completely sealed in a manner that will afford the same level of integrity as the geologic environment prior to drilling the borehole.
The ability to seal cavities and drill holes is reasonably well developed for seals which maintain integrity on a short-term basis. For example, existing seals have been in place for 10 to 50 years rather than the hundreds of thousands of years required for waste disposal. In general, present-day plugging materials are various cements and clays. However, many problems can be encountered if the plugging is not carried out properly. For example, temperature affects the chemical reaction and the resulting characteristics of the cement. Cement sleeves are subjected to progressively higher temperatures from mixing to pumping into the hole and final cement curing. The cements must be able to withstand the temperature encountered during pumping and the static temperature after the hole is filled.
The quality of the plugging is also influenced by the nature of the drilling mud used during the drilling operation since this mud lines the walls and will likely weaken the bonding strength of the cement to the formation. Therefore it is essential that all circulatable mud be removed before plugging begins, thus increasing overall costs of the operation.
In some situations, such as opposite fresh water zones, casings are left in place and filled with cement; however, in time the casing may erode away, allowing a path to man's environment.
The only positive method to neutralize the boreholes is to restore the stratum to essentially its original strength, permeability and compatible chemical characteristics.